1. Technical Field of the Invention
This invention is concerned with real-time demultiplexing of channel-sequential data samples as applied to multichannel seismic data acquisition systems.
Definitions
Channel-Sequential Order. In a multichannel system, a multiplexer successively samples the signal present at each channel in sequence, during a scan cycle. The series of data samples acquired from the respective channels during any one scan cycle constitutes a scan of data samples, or more simply, a scan. The resulting data samples in each scan are arranged in the same order in which the channels are sampled. Thus the sequence of data samples will be:
Ch-1, Smp-1; Ch-2, Smp-1; Ch-3, Smp-1; . . . ; Ch-m, Smp-1; PA1 Ch-1, Smp-2; Ch-2, Smp-2; Ch-3, Smp-2; . . . ; Ch-m, Smp-2; PA1 . . , . . . ; . . . , . . . ; . . . , . . . ; . . . , . . . ; . . . , . . . ; . . . , . . . ; PA1 Ch-1, Smp-m; Ch-2, Smp-n; Ch-3, Smp-n; . . . ; Ch-m, Smp-n. PA1 Ch-1, Smp-1; Ch-1, Smp-2; Ch-1, Smp-3; . . . ; Ch-1, Smp-n; PA1 Ch-2, Smp-1; Ch-2, Smp-2; Ch-2, Smp-3; . . . ; Ch-2, Smp-n; PA1 . . , . . . ; . . . , . . . ; . . . , . . . ; . . . , . . . ; . . . , . . . ; . . . , . . . ; PA1 Ch-m, Smp-1; Ch-m, Smp-2; Ch-m, Smp-3; . . . ; Ch-m, Smp-n.
Data words acquired and recorded in channel-sequential order by sample number are said to be arranged in multiplexed format.
Multiplexer. A switching device having a plurality of inputs and a single output. During one sample interval, the multiplexer will scan each input channel in sequence, sample the signal there present, and deliver the sampled signal through an output bus to a signal processor. A sample and hold circuit is assumed to be incorporated into the multiplexer.
Memory Element. For purposes of this disclosure, a memory element is considered to be a location in a memory of sufficient size to contain the four bytes that make up a data word. The memory elements are addressable in consecutive order.
Sample, Data Sample. A digital representation of the sign and magnitude of a sampled analog signal. Expressed as a series of bits, a sample may consist of as many as 32 bits, divided into four bytes of eight bits each. The data samples may be expressed in fixed-point or floating point notation.
Sample-Sequential Order. Data samples are grouped by channels with the samples for each channel arranged in order of the sample number. Thus, the sequence of data samples will be:
Data words recorded in sample-sequential order to channel number are said to be arranged in demultiplexed format.
Sample Interval. The time interval between successive samplings of the same channel. The sample interval may range from 1/4 millisecond to 4 milliseconds or more.
Scan Cycle. The time interval during which the signal present in each of a selected number of channels is sampled. The length of a scan cycle is equal to the sample interval.
Recording Cycle. The time interval during which the signals present in the respective input channels are sampled and recorded. Commonly, the recording cycle may be 8 to 16 seconds long. At a sample interval of two milliseconds, 4000 samples will be gathered from each input channel over a recording cycle of eight seconds.
Trace, Trace-Sequential. Data words in sample-sequential order. When converted to analog signals, such data word sequences are displayed as traces on a visual recording medium such as a seismogram. There will be as many traces on a single seismogram as there are data channels.
2. Technical Description of the Prior Art
Present-day seismic data acquisition systems may include more than one hundred signal input channels. The seismic signals present at each input channel are sampled periodically by a multiplexer at intervals such as one or two milliseconds (thousandths of a second) or at some multiple thereof. All of the channels are repeatedly scanned or sampled during a recording cycle of prescribed length. The data samples are processed and are then recorded on an archival storage medium in multiplexed format. For presentation as a visual display of underground earth layers, useful for geological interpretation, the data must be demultiplexed in sample- or trace-sequential order.
Traditionally, seismic data were recorded in the field on magnetic tape in multiplexed format. The tapes were than sent to a data processing center where the data were demultiplexed, further processed, and displayed on visual cross sections. Typically, the multiplexed, field-recorded data were read into a first mass-memory storage device such as a magnetic disc. Demultiplexing was performed by selecting the first sample from each channel from the first storage device and storing the respective samples in a second storage in locations that are separated from each other by a selected number of sequential address slots. Additional samples from the respective channels are then stored in locations that are shifted one address position from the corresponding previous address position. The selected number of sequential address slots is of course, equal to the number, plus one, of channels to be accommodated. It is evident that a very large mass memory is needed since, for a 128-channel system, with a sample interval of 1 millisecond and an 8-second recording cycle, more than one million data words may be recorded per seismic record.
In recent years, the trend in seismic exploration has been to move much of the preliminary data processing, including demultiplexing of multiplexed data, to the site of the field operations. The data processing equipment must be mounted in a recording truck or, at sea, in a boat. Typically, bulk storage devices, such as magnetic discs are extremely bulky and somewhat delicate. Such devices prefer a benign environment, a condition not often found in the field.
As an alternative to a disc, a static memory may be used as bulk storage. But when an entire seismic record has been read into the storage, new data cannot be entered until the static memory has been emptied of the previous data. A considerable amount of lost time results. It would, of course, be possible to provide twin bulk storage units; new data could be written into one unit while old data is being read from the other unit. This practice doubles the cost of the preprocessing equipment as well as its physical volume.
One method for demultiplexing seismic data is disclosed in U.S. Pat. No. 4,016,531. In this system, a magnetic disc is used. As a teaching of the necessity for demultiplexing multiplexed seismic data, this patent is incorporated herein by reference. Other teachings of the use of a magnetic disc for use in demultiplexing and preprocessing of seismic data will be found in U.S. Pat. Nos. 3,883,725 and 3,930,145. The objections cited above, of course, apply to these known prior-art systems.
In a related, concurrently filed application Ser. No. 946,897 and assigned to the assignee of this invention, a demultiplexing method is disclosed wherein a static memory is employed. The number of locations in the memory is determined from the product of the number of channels and the number of samples to be demultiplexed, plus an additional initialization buffer. The dimensions of the initialization buffer are related to the ratio between the sample loading or storage rate and the sample extraction rate and to the ratio of the number of channels to the number of samples per channel. But the addition of an initialization buffer represents additional expense and bulk. Accordingly it would be desirable to make use of a real-time demultiplexing scheme that requires no more than just enough storage capacity to contain the data from a single recording cycle.